Max Q

Max Q: Maximum dynamic pressure; the point during launch where the vehicle is subjected to its greatest aerodynamic stress. 

Max Q is also the point in the launch sequence where the rocket experiences the maximum dynamic pressure. This is the part of flight where the rocket is most likely to become damaged or break up.  

A space rocket, as it is going up into Space, is experiencing three forces: gravity (down), thrust (up), and atmospheric drag (down). 

The rocket starts to accelerate, and the drag force starts to grow dramatically, since the force is related to (velocity squared): whenever the speed doubles, the force quadruples. But, at the same time, the air density is decreasing, so the force is weakening because of this.  

At a certain altitude, the decreasing air density wins out over the increasing velocity, and the drag force starts to decrease. It is this point – where the increasing velocity is balanced by the decreasing density of the atmosphere and the drag force starts to decrease – that is called Max Q. 

When the spacecraft reaches an altitude of about 11 km (36,000 ft) and a speed of 1,600 km/h, dynamic pressure reaches its peak of 3.3 tons per square metre. It is at this moment, a very specific intersection of airspeed and atmospheric density, that we find our answer. This is ‘Max Q’. 

So how does a Rocket cope with this? The rockets throttle down at Max Q. 

Exactly ALL of that pressure is coming from the rocket’s thrust and the thickness of the atmosphere. Since they can’t change the atmosphere, they simply throttle down until the air starts to get thinner. Then they can throttle back up. 

Check out this video: What is Max Q | Max Q Rocket Science Explained